The welding of small metal parts by resistance is used in many different industries. It is applicable for the surface joining of parts and in the welding of plate-like elements to be bonded together.
The classical welding operation consists of placing two parts between two electrodes applying pressure with a specific force at the moment of passing a defined electrical current for a specific time between the two electrodes. The considerable resistance that exists between the surfaces in contact with the two parts and the high current that passes produces a high degree of heat that melts the surrounding material, thus making the weld.
In many cases two parts of differing sizes are welded, such as a nut onto the surface of a sheet of a certain size. The large part is usually positioned below and the small one is loaded by hand, by means of shuttles or pistons, onto the large part prior to applying of the pressure with the electrodes to both parts and welding.
In this method of welding the time parameters for the circulation of the electrical current and the pressures exercised by the electrodes onto the parts to be welded are important.
In turn in the case of wanting to introduce a welding step in a progressive machine press or in a transfer station, this must be carried out at the end of the process and not in an intermediate position, as would be the most logical. This is due to the fact that during the pressing cycle a pair of electrodes facing each other is not in a position to be able to carry out the resistance for the time necessary with the correct pressure. This problem is commonly solved by means of fixing the parts to be joined by means of a rivet or something similar. In effect the riveting operation is adapted to the rising and falling cycle of the dies in a progressive press; however a rivet has low resistance to the traction and zero resistance to the torsion.